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by Mal Marsden - 06/16/22 7:00 pm
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Allan G, davidhernandez, gavin eisler, gunner, JER.Hill, Mark Parker, NickL, pushrod tom
Total Likes: 19
Original Post (Thread Starter)
by Mark Parker
Mark Parker
We will find out if I get this right. Both have 38mm carbs and we are going to change them to 34mm. Both are 90degree. The first one, my brother's has a Devimead cylinder and 80mm Ed V forged pistons. The flow through that older big valve head was around 172cfm with a raised floor. Ben's was done years later and wider near the guide, also with a raised floor though oval. So it flows more. It has an old alloy cylinder with liners and uses Paul's old Devimead pistons.

It measured 9.6-1 and the cylinder is about .8mm too long. With the steel rods .040" piston to head should be fine, and if the combustion chamber is the same it should be 10.52-1 with it shortened .8mm, though 11-1 or a bit more would be good if we can get it.

Tony Price used 44.5mm B50 valves in his 734 racer with oval ports and 32mm Concentrics with devastating effect. He used 11.5-1 on pump petrol. In those days Dave Bennallick had a superfast Triumph 8v, his engine was equal to the BSA up to 90-95 but by 100 the BSA was moving on. The advantage of the BSA was it just kept going. People moaned about it being a big engine and finally protested it and had it measure... 734. Triples grew to over 900cc to beat it.

These heads have nice nitrided Jaguar valves that are 44.5mm. So now the heads for these are reading over 180 and they are higher speed. And we will see how they go. Previously Ben's had a power curve like a hot Triple with lots of go at high rpm, we want that but we want the big midrange punch to go with it. The vibration on the 90s does not hamper rpm. The 9.6-1 wasn't really doing it any favours. My 883 runs 11-1 on pump fuel. And I have run 12-1 without knowing, but when I did know I chickened out and fitted a second head gasket.

Stock bikes can ping on 9-1 yet the Firebird does not, no matter how I abuse it, I can make it pull from 2,000 or a bit lower in higher gears, I used to think it was stock piston shape compared to the B44 pistons, but maybe how the mixture behaves when entering effects it as well. I can get 91-95 and 98 fuel, I've been using 95 in the Firebird. It would be interesting to try it on 91 but I own it.

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Liked Replies
by Mark Parker
Mark Parker
This might be interesting, I was looking for old data graphs on my rgvbsa. Probably on a dead computer. When I started it was 818cc with Norton crank and T140 pistons. It had big valves and I struggled to get 48hp though I thrashed repeatedly with 38mm vm mikunis. Trying to get the jetting. So I put 38tms on it and heaps of runs later had 52 then a best of 55 it's shown to be similar to rwhp. So 62hp at the crank and it would hold that to 7,500. Then I made an alloy big bore kit 883cc and started at around 62 on the data logger. Heaps down low went dead around 6,000 and probably peaked before that. I fiddled with stuff and tried different heads to get around 72 or around 81 at the crank, though I didn't realize it's readings were like rwhp. Especially struggling running it to 7,500 for 55.

So this run is probably around then it's a graph of rpm in first and second with the 4 speed.

This is just rolling on the throttle, you can see the thing come on song when power comes on in first and the graph gets steeper, second isn't as steep because of the effect of wind resistance. The dyno function works with accelerometers plus weight, speed, time etc. So 1st gives the most accurate graph, more compression lessens that dip at lower rpm.

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The second graph I just found shows a run with 12-1 compression and the head altered again, around 80hp on the data graph and torque around 68ftlb similar to at the wheel. That's pretty massive actually, I hadn't looked at that before 77ftlb at the engine if it's right. I'd rebuilt it with one new case half and filed the cases for ages to be square and thought I was around 10-1 without measuring. And got a shock when I did. So I put 2 head gaskets on next time I had it off. So it has 11-1 now and a better head that gives more top end but torque is down 4ftlb by comparison, though stays longer at rpm.

These rollons are the same bit of road, once at rpm it's not much difference but the response is better with high comp. Because cruising in top means around 3,000. It doesn't particularly lack response there but more compression can boost that range. 4,800 in top is just over 100. 7,000 in second is 103 with the C/r 4 speed. With the 5speed both 1st and second are a little lower.

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3 members like this
by DMadigan
This is the starter installation in printed form:
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A short video showing the idler gear dropping into the clutch gear is here:[email protected]/51500953263/in/album-72157688652393165/
I could not figure out how to get a link to the video from Flickr, only download. The solenoid action goes through a rocker to rotate the idler down rather than move the Bendix out as is normal.
This is the clutch that will use KTM plates, pressure plate, Belleville spring and spring retainer.
KC kindly gave me a big plate of aluminum to make the spacer for the primary. I am going to narrow a damaged early cover to put over it. Eventually both will be cast.
The XS crank rod centres are the same as the A65 with the single ball bearing in the middle. There are spacers on either side of the bearing to control the press.
I already have the tool paths for remachining the XS timing side into an A65 pattern. I decided to keep the metric bearing on the drive side so made a triplex sprocket to match.
2 members like this
by Mark Parker
Mark Parker
My 650 starts so easy, and 750s as well, the 883 needs decompressors and a hill or roller starter in the winter, and probably a sacrificed rat.

Anyway the big awaited test. Calibrate and one after the other. It's colder here today and raining so calibration is different but its the comparison that is the thing. Cannot calibrate for speed just compare. At the carb needle seems a good spot

So the 38mm head we took off, 185cfm through a bell.
179cfm through a 38mm Lectron, it had 38TMs and they will flow a bit less, but 23"w speed at the needle or 320fps for a Lectron, which is actually regarded as very good.

I stuck a straight 34mm manifold on it (not a brilliant match but ok) bell 187.75cfm 34mm pwk 176cfm and 34"w or 380fps.

The new 34mm head that's ready, with longer curved manifold, bell-183.4cfm 34mm pwk 178cfm (I know it doesn't make sense but that's what it measured) and 380fps at the needle, this speed is F1 4valve speed. To get that through the 38mm requires much better efficiency so probably 220cfm or so, which it cannot use anyway.

So basically the 38s are probably less or the same maximum flow, but must rev more to get the speed it needs to respond, but it still cannot overtake the 34 ever, that kinetic energy pushing charge in will always be less. At high rpm the little one will still kick its butt, just how severely I don't know. But more compression compounds it. So it should be fun. Can anyone follow this? I think that it's, and the more instant gas speed that does it, with big volume as small volume limits it. This is better than a new Triumph 4v if the flow is ballpark and 34mm compared to Triumph's ? port size and speed?

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2 members like this
by Mark Parker
Mark Parker
Back to Paul's 80x74. With the crank out for the first time since the 1990s. This experimental crank is a bit different to the last two. It's lighter and not quite as smooth. The added counter weights on the flywheel are smaller with only two bolts. And the flywheel is drilled to lighten on the other side. The crank is std on the big ends and they look fine. As do the main bearings. Tempted to re-use the mains except how many miles it has done. Rotor is loose on the steel centre and needs a new one.

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Rods have been used since the early '80s. I'll try getting it dynamically balanced with new rods and see if it can be smoother. It may be a smart thing to make 90 degree cranks closer to std weight. If they vibrate less. People add balance shafts to get smoothness and that is more serious weight to spin up.

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2 members like this
by Mark Parker
Mark Parker
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The crank can have any pin leading, SRM have cams but with the Timing side pin advanced 90degree. If it's like here you need to have one made.
It can be lighter but it isn't an advantage, hp makes it respond far more that a lighter crank, which means more vibration.

To line it up I use long screw drivers that fit the holes neatly and you can line them up with the crank. Though good machining means the assembled shaft's alignment, so you don't end up with an 89degree or worse pin, the screw drivers can be sighted. It's very obvious when they are a bit out of line. The 74mm stroke and 80mm pistons make lovely engines. The factory forged Trident R3 cranks they could have easily done a 90degree. This has more bolts than a Commando crank and a bigger flange and bolt circle, but some parts get a bit thin.
1 member likes this
by Mark Parker
Mark Parker
I think the smoothest one is around 50% with steel rods, it may be a bit heavier in the flywheel. Than the first, though the first has much higher compression. But the first was balance statically not dynamically, and I'll try getting that done while it's apart.

.8mm milled off the cylinder. It shows why I prefer Nikasil. This old one has thin liners and they sit on a step at the bottom of the bore. The head studs are 9mm 1.25thread, and very close to the liner, they are hardly common but some VWs have 9mm head bolts we modify. I originally used 8mm but with rising hp they started pulling out on my Nicasil cylinder. I considered these cylinders scrap and practiced drilling and tapping the 8mm to 9mm. Drilling by hand as it was more accurate to feel the drill going straight though I tried a coupe on a press that wasn't very successful. I didn't take much care.

Then Ben wanted to use them and they are straight enough. I think the centre one is still 8mm. They are all down 35mm or more. The rear two can be 10mm but not the rest.

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So this is a shortened cyl for the 74mm stroke and we didn't have it quite right.

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If it has clearance maybe we can make a .5mm head gasket, for over 11-1, but 10.5 will be much better. Plus the faster gas speed will hopefully do what we hope.

I cannot imagine Tony Price's 734 not having 80hp or more.
1 member likes this
by NickL
Peter Brown was the kiddie with the a65 heads i think Mark, he probably
let a few people know about it. Unfortunately i wasn't one of 'em...............
My stuff was all home brewed although it worked quite well, i still used to
be amazed by Stu Digby's outfit, he was one of the ones in the know.
1 member likes this
by Mark Parker
Mark Parker
I messed around compared speeds yesterday and today. At the needle of the pwk comparing two 34mm heads one with 44.5mm valve and the other with 42mm. In inches of water in the probe it's 32" to 35-36" but the speed difference isn't that significant 32" is 370fps 36" is 390fps measuring in the same position.

The smaller valve is getting the job done.

I tried pushing the probe down passed the needle 42mm valve only, and it's 41-46"w down there, so over 400fps.

I think it's flowing smoothly, like a smooth bore carb. Without too much turbulence blocking it. So a vacuum of 28"w in the cylinder really gets the air hiking through the port.

According to Vizard's book on porting:
Stock 2 valve ----------------------- 280fps
High precision cast stock 2v --- 300fps
Fully ported as above ---------320fps
Race non production 2 valves ----340-360fps @ 28"w.

A production 4valve------------------300-310fps
A fully ported production 4valve - 300-360fps @ 28"w.
And F1 ----------------------------------380-420fps.


BSA 42mm valve ----------------------370fps at the carb needle. Over 400fps down in the port.
BSA 44.5mm valve --------------------390fps

Speed represents efficiency because it means high flow for equivalent port area.

Half that test vacuum 14"w, simulating lower rpm; at the carb needle 280fps. And 320-340 down in the port. I think this is why hp can be increased at lower rpm. It's interesting if hp can be increased without the need for increasing rpm which is the normal method. More rpm means more air is burned, but more air being forced in means more is being burned, more is being compressed and it's also like having more compression and the engine sounds crisper.

I test at .384" lift, if I set to more lift generally it flows less. I've had them flow more at .350" sometimes, and can correct that or at least not lose flow going to .384". Removing the valve means it flows less. So the valve is effecting the flow.
1 member likes this
by Mark Parker
Mark Parker
This is the graph from Tom's dyno of his fairly stock Hornet with one of my 34mm carb head. It isn't making the higher rpm power I expected but the midrange is good.
Until I get data on the Firebird I don't know the comparison. Comparing with a graph on a Rickman 8v of similar engine size 683 to 680 has the BSA
behind by 1.6hp The Rickman has higher compression though at 11-1.

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This is what I'd like to figure out. The 883 is different with a tuned exhaust with X connector. It's 2" at the collector and tail pipes the headers have stubs at std size then step up 2 sizes. The science in it is experimenting and it's the best I've tried. The compression is 11-1 carbs 38tms that are modified a bit. Cam is std profile though to suit 90degrees with offset rocker buttons for .410" lift and a few degrees retarded.

So the 680 is .77 of the size. I would not expect the 680 to have .77 of it's hp. But I'd like to understand the difference, or what is making the difference because .77 of 82.5rwhp would be 63.5 at around 8,000. That's what we will be aiming at with Ben's 650 racer. Even drastically different dynos probably would not account for it as the curve on the 883 doesn't fall till close to 8,000.

The Firebird has std headers and a short balance pipe section I made plus two mufflers. The balance pipe is probably a little bigger than std. And I don't know if the power is better or not than Tom's, it's stock under the head and I use pwks not tms on it, if it makes a difference. I could stick a data logger on it and get a hp graph which might show when hp starts falling. Or try booking it in. Though I want to stop it buzzing so bad at rpm and put steel rods in.

This graph is on Tom's Hornet as stock with +.040" pistons. Monoblock carbs. Two runs with stock pipes giving 36.33 and 36.44hp and one with a 2 into1 giving 38.99hp, that curve has a dip in the middle but is stronger earlier and later.

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So using the same pipes the difference is 45.72rwhp to 36.44rwhp for stock. Converted to power at the crank as the factory measured is 41.33hp 41lbft std to 51.86hp 46lbft with the 34mm head.
1 member likes this
by Mark Parker
Mark Parker
Ben's been up spectating at Wakefield park. 2 Period 4 bikes only. They run them in unlimited superbikes up to '97. So really he could have used a cheaper to build RGV version. I guess there may be days of more P4 bikes. They are just expensive. And limited in many ways with tryes and wheels and forks and brakes. It will take a while to sort an OIF.

Talking to Ivan with a Yamaha 90degree750 twin. He ran it at 840cc and 360degree previously but it kept failing. He said there was no comparison with the 90degree. His friends used to say he was wasting his time and now most of them have done it as well. He said it makes around 70hp and is148kg. Why these older classes of racing are so interesting. Ben may have taken photos.

I think there is good reason to use the 74mm stroke to go road racing.

I think this is Ivan:
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1 member likes this
by NickL
Rather than splitting an a10 crank, you may as well use a norton one
and make a flywheel. My brother made several offset twin road bikes
for blokes here that way using t140 pistons. They were very good as
touring bikes. They came out at 810cc using standard t140 pistons and
the compression was quite low until the barrel was skimmed if using
norton rods.
THe norton crank is much stronger than the a10 one and would be
easier as a starting point. As an aside he also did several triumphs
that way too. The crank goes into a t140 quite easily.
1 member likes this
by Mark Parker
Mark Parker
That one is 80mm.

Bens back tonight describing the Norton which is also 90degree and the racing. He said the '80s model Pantah the Norton and Yamaha sounded very much the same.

I found out some interesting stuff actually. A 750 BSARGV? Na, I'd take the red one (883). It might blow up. It didn't when you were in hospital. (The boys took it for a ride and the SV.) The SV isn't slow and it's geared up a bit. With USD forks and good brakes, 5.5" rear wheel and good tyres. Yea, the SVs ok in the slow bits you can keep up, but anywhere the road opens up a bit you just go through the gears at max revs with the throttle against the stop and the red one just pulls away. I don't think he was revving it hard. The shift light blinks around 7,000 so probably had him change. But it pulls a lot of speed if you wind it up.

Anyway my thought is the 750 version can be tuned to go much the same.

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